1. Field of the Invention
This invention relates to a method for manufacturing a high tensile strength optical waveguide fiber in which a primary layer of a thermoplastic polyamide is applied to optical fibers after they are drawn, and while the polyamide is molten.
2. Statement of the Related Art
Optical waveguides consist essentially of glass fibers which have a refractive index profile such that incident light is guided in them around curves. Accordingly, they may be used as a light and/or signal transmission medium. The successful use of glass optical waveguides requires maintaining the high tensile strength of the glass fibers and avoiding increases in attenuation. To maintain the high tensile strength of the glass fibers, the optical fibers are sheathed immediately after drawing in at least one protective layer of plastic. The optimal layer thickness of the plastic film layer is determined by its critical mechanical properties, such as E-modulus and hardness, and is generally in the range from 10 to 100 microns (.mu.).
To avoid attenuation losses, the glass fibers have to be surrounded by a flexible material which does not show any phase transitions, particularly over the required in-use temperature range of -40.degree. C. to +80.degree. C. Thus, the glass transition temperature of the material must be &lt;-40.degree. C. while the change in modulus over the above-mentioned temperature range should amount to less than 2 powers of ten. Neither should there be any change in length at temperatures in the range from -40.degree. to +80.degree. C. This type of layer is called a primary coating. If required, another protective layer (i.e. a secondary coating) may be applied to the primary coating.
In the coating of optical fibers by lacquering, the bare fiber passes immediately after drawing through one or generally several coating units each followed by a drying zone. The coating units may be charged with non-reactive coating materials, i.e. polymers soluble in organic solvents, such as cellulose acetate, polyvinylidene fluoride or polyester imide. Hitherto, the use of commercial polyamides containing dimerized fatty acid has been curtailed by their inadequate low-temperature flexibility, because their glass transition temperatures are higher than -10.degree. C.
One of the disadvantages of the above-mentioned lacquering technique is that, in general, only very thin layers (approx. 5.mu.) can be uniformly applied in each coating cycle. Although a layer thickness of approx. 30.mu. per coating cycle can be achieved where thermally crosslinkable polysiloxanes are used, the protective film formed in this way is soft and critically lacking in mechanical strength.
In addition to the thermosetting films applied by lacquering, coating materials based on acrylates of epoxide, polyurethane and silicon prepolymers which can be crosslinked by shortwave light have recently been introduced. Although film thickness of from 20 to 50.mu. per coating cycle can be achieved with systems of this type, the aging behavior of these coating materials and their effect on the static fatigue of the optical fibers have not yet been resolved. The problems presented by the residual monomer content also have not been satisfactorily solved.